Add test failing on array types on PHP 8+

[Zvax]

After a discussion in r11 it appears that from this PR the behaviour changed so that in PHP80+ typehinting an array fails because getType returns the actual string "array" whereas pre-PHP80 the value was simply null.

Returning "array" causes auryn to try and instantiate an array, which is not a class, and triggers error 8: Could not make array: Class "array" does not exist.

Orthogonally, phpunit being at version 4 makes it incompatible with PHP80+, which is necessary to make the test fail. Allegedly this would be fixed by #188.

cmb in chat proposed a quick stub that could solve the issue. The changes do not seem to impact in any way the code from the phpunit upgrade PR so there's no merging order requirement.

[kelunik]

$type = $param->getType();
if ($type instanceof \ReflectionNamedType && $type->isBuiltin()) {
    return null;
}

return $type ? (string) $type : null;

I have that locally now in getParamTypeHint.

[Zvax]

Yes, this fixes the error here, I have put this in StandardReflector. I don't know how to test it with the caching reflector to make sure it works with it as well.